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New study on microbial air sampling accuracy finds Petri dish dimensions can impact results

publication date: Jul 20, 2023
author/source: Cherwell Laboratories Ltd


Cherwell’s investigation extension further explores variations in flow rate observed in Petri dishes used with microbial air samplers

Cherwell, specialists in cleanroom microbiology solutions for the pharmaceutical, healthcare and related industries, has published findings of a new impartial investigation into the impact of using pre-poured Petri dishes from different suppliers for air sampling in cleanroom environmental monitoring (EM) programmes [1]. This extends Cherwell’s initial ‘plate choice’ investigation which found that size and form factor of plates can affect microbial air sampling accuracy [2].

The original study was very well received at recent industry events, instigating wider discussion around sterile medicinal product manufacturing safety and subsequent questions. These have been followed up by the latest study, providing useful additional insights of significance for EM regulatory compliance.

The updated study report - “Comparative analysis of Petri dish variation in flow rate for air sampling” - delves deeper into the flow rate variations in prepared media plates observed in the initial study. It analyses the potential contributing factors, such as fill volume, petri size and agar height, and provides recommendations for ensuring consistent and accurate air sampling. It also serves as a means to share findings and stimulate further investigation in the field of air sampling and Petri dish variation which can impact Annex 1 compliance and the safe manufacture of sterile medicinal products.

The comparative investigation of Petri dishes from six different media suppliers identified the factors contributing to sampling variations by taking additional measurements from all plates. It highlights that pre-poured Petri dishes deviating from the recommended standards used for air sampling can cause flow rate variations. If these variations are not accounted for, then under- or over-sampling can occur, resulting in unreliable data.

The study report concludes that for compliance with industry guidelines, particularly the revised Annex 1, organisations must ensure the careful selection of appropriate Petri dishes and air samplers to achieve accurate and reliable air sampling results.

“Our initial study was definitely appreciated, and discussions around it with delegates at recent industry events led to further questions being raised. We didn’t want to leave these questions unanswered and so carried out further investigations detailed in our latest study report,” said Hamish Hogg, Microbiology Product Specialist, Cherwell. “These studies have generated such interest that our microbiology laboratory team now plans to carry out more investigations of a similar nature. We’re very open to suggestions of what to investigate so we can continue to share useful knowledge which can support the safe manufacture of sterile medicinal products.”

The new microbial air sampling study - Comparative analysis of petri dish variation in flow rate for air sampling – can be downloaded on Cherwell’s website



  1. Cherwell Investigative Study. Comparative analysis of petri dish variation in flow rate for air sampling. Published July 2023.
  2. Cherwell Investigative Study. Does plate choice change sampling accuracy? Published February 2023.



About Cherwell Laboratories

Located in Bicester, central England, Cherwell Laboratories provides standard and bespoke cleanroom microbiology solutions that help customers effectively manage their controlled environments and processes. Established for 50 years, the company has built a strong reputation as a leading supplier of prepared microbiological media and environmental monitoring equipment, with considerable expertise within the pharmaceutical and related industries.

Cherwell manufactures its extensive range of Redipor® prepared media products in its ISO9001 registered facility, and has the flexibility to respond to varying customer requirements - from routine high volume down to low volume specialist applications. In-house engineering facilities support production and the development of bespoke solutions, as well as the supply, service and calibration of its environmental air monitoring equipment solutions.

Cherwell builds strong partnerships through professional, responsive and open communication, and by delivering high quality products, plus a flexible and reliable service with expert support, Cherwell ultimately helps its customers to reduce risk.


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